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Fever refers to the manifestation of heat on the body surface. Most diseases involve fever; heat is generated by Fire (Huo), and Fire originates from Qi (vital energy). (Zhu Danxi stated that "excess of Qi constitutes Fire," illustrating this principle.) The underlying mechanism lies in two aspects: "Qi deviation" (Qi Guai) and "Qi stagnation." (Fever caused by smallpox/rash in infants is discussed separately.)

The first type is Hyperactive Yang Fever.
Yin, Yang, Water, and Fire are originally in harmony, neither hot nor cold, which is termed Righteous Qi (Zheng Qi). Once deviation occurs, there will inevitably be an excess of one side. The *Classic* states: "Yang excess leads to heat." This is hyperactive Yang Fire. Symptoms include irritability and thirst, dry constipation, dark and scanty urination, and a surging and rapid pulse in all six positions. Treatment should employ cooling methods. (Some cases arise from the accumulation caused by heavy wine consumption and rich foods; some result from contracting dryness during hot seasons; others stem from extreme emotional excess leading to hyperactive and violent Heart Fire.)

The second type is Yin Deficiency Fever (Blood Deficiency Fever is similar).
Yin deficiency refers to the depletion of Kidney Water. The nature of Fire is to flare upward and manifest externally; when restrained by Water, it achieves the interaction between Li (Fire trigram) and Kan (Water trigram), like a dragon hiding in the abyss, nurturing Spirit (Shen) internally and facilitating communication (Jiao Tai) downward. If Yin fluid is depleted and Kidney Water is insufficient, it is like dry firewood with intense flames, causing burning disaster. (This may be caused by excessive sexual activity damaging essence, diarrhea or dysentery losing Yin fluids, or dry-heat injuring body fluids.) This flaring fire is caused by water deficiency, differing from the previous category where Yang is hyperactive but Yin is not yet deficient. Symptoms include dry mouth, emaciation, poor appetite and lassitude, intermittent headache, seminal emission and night sweats, bone-steaming fever with muscle wasting, red lips and malar flush, coughing with blood-stained sputum, eventually developing into Lao Zhai (consumption/tuberculosis). Treatment should use sweet and moistening agents to nourish Water to control Fire. If bitter-cold herbs are mistakenly used, the Fire will be constrained by Cold, causing its eruption to become even more violent. According to a case treated by Xue Lizhai: An elderly patient suffered from Kidney deficiency with Fire failing to return to its source, wandering externally, presenting with fever, irritability, excessive thirst, red face and eyes, prickles covering the tongue which shrank like a lychee, cracked and scorched lips, sensation of smoke/fire rushing up the throat (relieved slightly by drinking cold tea), soles of feet feeling like iron plates, copious phlegm and saliva, frequent urination, wheezing and urgency, surging and rapid pulse without regularity but forceful, and a scorching body upon touch. Xue Lizhai treated him with Shiquan Dabu Decoction (see Deficiency Damage chapter) added with Cornus Fruit (Shanzhuyu), Alisma (Zexie), Moutan Bark (Danpi), Chinese Yam (Shanyao), Ophiopogon Root (Maidong), Schisandra Berry (Wuweizi), and Aconite Root (Fuzi), along with Bawei Wan (Eight-Ingredient Pill, see Deficiency Damage chapter), resulting in recovery. I believe this case clearly demonstrates Water deficiency with Fire flaring, for which Liuwei Dihuang Wan (Six-Ingredient Pill, see Deficiency Damage chapter) should have been used. The reason for using the aforementioned drugs was likely due to the patient's advanced age, indicating deficiency in both Water and Fire. If Fire were not deficient, one must not mistakenly use warming herbs based on this case, as it would further injure the Yin aspect. (In deficiency patterns, regardless of Water or Fire, the pulse is typically weak; however, the forcefulness here was likely provoked by previous administration of cold herbs. Therefore, when using cold herbs to directly subdue Fire, they must be taken warm. If ineffective, the herbs should be taken after being processed with ginger juice or wine fried, which leads to recovery. This also prevents constraint. Furthermore, this condition resembles Excess Fire, but Deficient Fire is often more violent than Excess Fire because it floats outward having lost its root, fully manifesting externally, unlike Excess Fire which has internal heat penetrating outward but hides its brilliance and flame. *Yiren Bian* records: There was a case of mistakenly taking Baihu Tang, leading to intense heat like burning, radiating three to five chi, making people unable to approach. This illustrates the violence of Deficient Fire.) Blood Deficiency Fever may result from hematemesis, epistaxis, melena, postpartum metrorrhagia, or any blood loss. Symptoms include irritability, dark complexion, and incessant thirst. The presentation resembles the Baihu Tang pattern, distinguished only by a pulse that is not long or solid, but floating, large, and empty upon deep pressure. Mistakenly administering Baihu Tang is dangerous. Treatment should focus on nourishing Yin and tonifying Blood. If Yang is also deficient, Qi-tonifying herbs should be added; "for blood loss, tonify Qi," as the generation of Yang promotes the growth of Yin.

The third type is Yang Deficiency Fever.
Yang deficiency refers to the weakness of Kidney Fire (Mingmen Fire). Yang deficiency should lead to cold intolerance; why does it present with fever? This is because there is deficiency accompanied by Cold. (If there were no cold signs and only Yang deficiency, it would merely show insufficiency without fever.) Cold inside repels Yang to the outside, causing external heat; (Yang is rejected by Cold, exits to the muscle layer, and if it floats outward without collapsing, yet cannot return inward, the struggle generates heat.) Cold below repels Yang to the top, causing upper heat. This is Rootless Fire, merely a false flame. Symptoms include irritability, a desire to sit or lie in mud/water, a flushed face resembling mild intoxication or shallow redness on the malar bones that shifts position (unlike the full-face redness of Excess Heat), thirst with desire to drink, or sore throat where water is requested but cannot be swallowed. Although the body surface is very hot, deep pressure reveals no heat or even coldness, and the feet are invariably cold. Urine is clear and profuse, stool contains undigested food (Qinggu), and the pulse is deep and fine, or floating and rapid but weak, dissolving upon pressure. Treatment should employ warming agents. Warming the Middle Jiao returns Yang inward; warming the Lower Jiao returns Fire to its origin. Mistakenly administering cold herbs leads to immediate death. (This may occur if one consumes cold food or inhales cold air; in individuals with pre-existing Yang deficiency and Yin excess, external cold invasion allows Yin pathogens to flourish, causing True Yang to lose its hold.)